Manufacturers of rubber products long have dispersed fillers into various polymers as a means of providing both physical reinforcement and bulk; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04. These compositions often contain ˜30% by wt. reinforcing filler, which have a great effect on properties as tensile strength, abrasion resistance, and tear and cut resistance of the rubber compound.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat buildup during operation of the tire. Reducing the buildup of heat in a tire during operation is a primary target for increasing fuel economy of automobiles because a significant portion (on the order of 20%) of the energy derived from combusting gasoline is needed merely to overcome this rolling resistance. Reducing the inherent rolling resistance of the tire means that less fuel is consumed and, concomitantly, less CO2 is emitted.
The foregoing considerations are, to a great extent, competing and somewhat contradictory: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties.
The first material commonly used as a filler was carbon black, which is known to impart good reinforcing properties and excellent wear resistance to rubber compositions. However, carbon black-containing formulations often suffer from increased rolling resistance. To alleviate this, some efforts have focused on reducing the amount (i.e., volume) and/or increasing the particle size utilized, both of which typically entail some deterioration in reinforcing properties and wear resistance.
Over the last quarter of the 20th century, amorphous silica and various treated variants thereof grew in use as a filler, both alone and in combination with carbon black. Use of silica fillers in tread stock can result in tires with reduced rolling resistance, increased road traction on wet surfaces and other enhanced properties. Silica-containing rubber compounds have gained wide acceptance in the manufacture of tire treads, particularly for passenger vehicles.
Nevertheless, consumers, regulatory bodies and vehicle manufacturers continue to demand ever-better vehicle tire performance; accordingly, alternative fillers continue to be investigated.
Aluminum hydroxide, although not as good as carbon black with respect to reinforcement, can impart favorable rolling resistance and wet traction properties. Examples of rubber compounds employing Al(OH)3 as a filler can be found in, e.g., U.S. Pat. Nos. 6,242,522 and 6,489,389. See also H. Mouri et al., “Improved Tire Wet Traction Through the Use of Mineral Fillers,” Rubber Chem. and Tech., vol. 72, pp. 960-68 (1999).
Examples of other alternative particulate fillers include metal oxides having very high densities (see U.S. Pat. No. 6,734,235); magnetizable particles such as iron oxide or strontium ferrite used in the manufacture of tire sidewalls (see U.S. Pat. No. 6,476,110); macroscopic (e.g., 10-5000 μm mean diameter) particles of hard minerals such as alumina, CaCO3, and quartz (see U.S. Pat. No. 5,066,702); pumice containing no less than 30% by wt. SiO2 and having a JIS-A hardness of 55-75 (U.S. Publ. No. 2004/0242750 A1); ZnO particles having a diameter of less than 0.01 μm (see U.S. Pat. No. 6,972,307); and ZnSO4, BaSO4 and/or TiO2 with average particle sizes of ˜0.5-1.0 μm, ˜1.0-2.0 μm and ˜0.05-1.0 μm, respectively (see U.S. Pat. No. 6,852,785). More often, potentially useful fillers are merely strung together in list format; see, e.g., U.S. Pat. Nos. 4,255,296 and 4,468,496 which mention silicic acid, CaCO3, MgCO3, talc, FeS, Fe2O3, bentonite, ZnO, diatomaceous earth, white clay, clay, alumina, TiO2, and carbon black.
In addition to using alternative fillers, some have sought to enhance dispersion of reinforcing filler(s) throughout the elastomeric material(s), which both enhances processability and acts to improve certain physical properties. Dispersion of fillers can be improved by increasing their interaction with the elastomer(s). Examples of efforts of this type include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer(s).
Physical properties that are sought to be impacted through improved filler dispersion include reduced hysteresis (as evidenced through lower tan δ values at elevated temperatures, e.g., 50° or 60° C.) and improved wet traction (as evidenced through higher tan δ values at 0° C.). The latter has the potential to negatively impact the rolling resistance of a tire at low temperatures, however.
The surfaces on which most tires are used tend to be made from Portland cement concrete or asphaltic concrete (asphalt). Concrete is a construction material that consists of the reaction product of Portland cement (a mixture of oxides of calcium, silicon and aluminum made by heating limestone with clay and grinding the product with a source of sulfate such as gypsum), construction aggregate (generally gravel, sand, crushed stone, etc.) and water; in recent years, fly ash, blast furnace slag, silica fume, and the like have been used in addition to or in place of some of the cement. Solidification occurs through hydration of the cement (and/or replacement material) which bonds the other components as it hardens.
Asphalt, like Portland cement concrete, is a composite material which consists of a binder and mineral aggregate; however, instead of hydrated cement, a bituminous material acts as the binder. The components are mixed together and then layered and compacted.
Both concrete and asphalt thus contain large amounts of minerals. These must be taken into account when attempting to understand the sliding friction between a tire and a road surface. Specifically, these materials constitute an extremely large fraction of the surface with which the rubber and/or the dispersed particulate filler must interact.
Neither friction (e.g., traction) nor wear resistance is an intrinsic property of tread rubber compounds; instead, they result from tribological systems of tread rubber, road surface, and any interposed materials such as water, ice, dust, etc. These systems involve an extremely complex matrix of variables including load, sliding speed, temperature, bulk viscoelasticity, interfacial interactions (e.g., adhesion and de-wetting transition), physicochemical characteristics of road surface, lubrication conditions, and the like.
Perhaps due to this complexity, friction of rubber articles on wet surfaces has been the subject of comparatively little published research, particularly given the fact that the subject has been a significant concern for nearly 100 years.
A leading article, published in May 1968, suggests that friction of rubber on wet surfaces is determined entirely by energy losses (hysteresis) produced in the rubber when it is deformed by the hard surfaces over which it slides and that any friction effect due to adhesion can be ignored. Much conjecture along this line remains; see, e.g., B. N. J. Persson et al., Phys. Rev. B 71, 035428 (2005) in which the smoothening of a surface due to water pooling, which effectively shrinks the effective size of surface asperities thereby reducing the friction contribution due to viscoelastic deformations of the rubber induced by such asperities, is suggested to explain the reduced traction seen on wet but rough surfaces.
A predictive method for identifying and utilizing efficacious rubber formulations would be highly desirable, particularly one which might identify formulations that could be used to provide articles (e.g., tire treads) exhibiting good wet traction performance.